If it is still registered to an account that means the current service provider (ADT) would need to cancel the Alarm.com account associated with it to let you use it again.
Most companies will freely do this, keeping the panel registered typically incurs some cost.
If they won’t work with you or release the panel it would need to be replaced.
It is very likely that they use the standard 319.5 IQ Panel 2+ models, but there is also a 433 and 345Mhz model. To make sure you choose the correct replacement, what are the model numbers of a couple of your alarm sensors? This can usually be found on the back of the housing or inside the cover.